Replace "constants" module with "resources"

These aren't constants and they were never meant to be.

I've also started collecting the UI theme values. For now, just button
colors.

src/resources.rs

@@ -0,0 +1,34 @@
+//! Program constants & defaults
+
+use bevy::{color::palettes::css::*, prelude::*};
+
+#[derive(Debug, Reflect, Resource)]
+pub struct UiTheme {
+  // TODO: Panes
+
+  // Colors for the "Big Red Buttons" (the main actions of the machines)
+  // normal
+  brb_bg: Color,
+  brb_border: Color,
+  // hover
+  brb_hover_bg: Color,
+  brb_hover_border: Color,
+  // pressed
+  brb_pressed_bg: Color,
+  brb_pressed_border: Color,
+
+}
+
+impl FromWorld for UiTheme {
+  fn from_world(world: &mut World) -> Self {
+    Self {
+        brb_bg: RED.into(),
+        brb_border: DARK_RED.into(),
+        brb_hover_bg: PINK.into(),
+        brb_hover_border: RED.into(),
+        brb_pressed_bg: GREEN.into(),
+        brb_pressed_border: DARK_GREEN.into(),
+    }
+  }
+}
+